The present invention relates to sintering methods for silicon nitride and sialon material compositions and in particular sialon materials useful for machining of heat resistant alloys.
Silicon nitride is a highly covalent compound with a number of interesting engineering properties. An adverse effect of the strong bonding is a low self diffusivity why the material cannot be consolidated by solid state sintering. Sintering additives such as Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and MgO are used to form a liquid with the SiO.sub.2 which is always present on the surface of the Si.sub.3 N.sub.4 grains. The resulting material has a two-phase microstructure consisting of silicon nitride grains embedded in an intergranular bonding phase, which is normally a glass.
Beta-prime SiAIOM (Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z) is a solid solution where 0&lt;z&lt;4.2. SiAIOM materials normally also contain an intergranular bonding phase which can be a glass or contain various crystalline phases. SiAIOM ceramic cutting tools may also contain alpha SiAIOM ((Si,Al).sub.12 M.sub.x (O,N).sub.16 where x is between 0.1 and 2 and M can be Li, Ca, Mg, Hf, Zr, Ce, Y, Sc or other lanthanides) and intergranular phases.
Silicon nitride and SiAION decompose at high temperatures which is why sintering normally is performed in nitrogen atmosphere.
Pressureless (atmospheric pressure) sintering, besides nitrogen atmosphere, normally requires embedding of the object to be sintered in a suitable powder bed to avoid decomposition.
Hot isostatic pressing requires encapsulation of the object to be sintered prior to sintering.
Gas pressure sintering (GPS) is a method of sintering powder metallurgical parts to almost 100% relative density without any of the precautions necessary for the above mentioned sintering methods. According to this method the first part of the sintering is performed at conventional pressure. When closed porosity is reached the gas pressure is substantially increased and maintained during cooling.
Another advantage of increased pressure during sintering, if nitrogen is used as pressure medium, is a reduction or elimination of the thermal decomposition of the ceramic material.